doc: remove invalid link to scikit-learn in index.

The documentation of the latest version of scikit-learn does not contain
a 'tutorial' page anymore. Removing the now broken reference to it.

doc/index.rst

@@ -17,8 +17,7 @@ Easily boost your :any:`sklearn.pipeline.Pipeline` with powerful features, such
 .. warning::
 
     Before any investigation of this package is capable of, check the scikit
-    learn :ref:`user guide <scikit-learn:pipeline>`. Several :ref:`tutorials
-    <scikit-learn:tutorial_menu>` are available online.
+    learn :ref:`user guide <scikit-learn:pipeline>`.
 
 .. warning::